The present invention generally relates to an impeller of the type used in a device such as a centrifugal compressor. More particularly, the invention relates to the configuration of the blades of the impeller which work on the gas supplied to the inlet or eye of the impeller as the latter is rotated to effect a change in the pressure and flow rate of the gas. In the operation of a centrifugal compressor of this type, the gas enters the eye of the impeller in an axial direction and follows passages defined between the blades of the impeller and a fixed casing located adjacent the impeller so as to be turned to flow outwardly in a generally radial direction and exit at the periphery of the impeller at an increased pressure or head. For efficient operation of the compressor, the flow of gas through the passages should be smooth and continuous with the passages being shaped so that turning and acceleration of the gas are as gentle as possible. Depending upon the operating speed of the impeller and the difference in the inlet and outlet pressures of the gas flowing through the passages, it may be desirable to provide the impeller with an inducer section so as to assure proper flow of gas through the impeller. Such as inducer section usually is in the form of one or more blades rotatable about the same axis as the impeller with the inducer blade or blades being wound in a generally helic-like fashion concentric with the axis. As its leading edge, the working surface of the inducer forms an acute included angle or inlet angle with respect to a plane transverse to the flow of gas. In this way, as the inducer is rotated, it helps direct the gas axially into the impeller for efficient operation of the impeller. While the inducer blades may be separate from the impeller blades, such as is shown in U.S. Pat. No. 3,299,821, on the other hand, the inlet end portions of the impeller blades may be curved in a generally axial direction to provide the inducer function such as is shown in Canadian Patent No. 212,667. The general construction of the exemplary form of the present invention is of this latter type.
In the design of an impeller having blades configured to provide both the inducer function and the usual pressure increasing function, typically, the speed of rotation of the impeller for maximum efficiency is selected for the desired output flow capacity of the compressor and the desired increase in pressure or head of the gas. This speed selection is based on theoretical and empirical considerations known in the art and outside the scope of the present invention. With speed set, the inlet eye diameter of the impeller and the inlet angle of the inducer sections of the impeller blades are determined for maximum efficiency under the operating conditions of the compressor.
Because the impeller speed, eye diameter and inducer inlet angle vary for different operating conditions, rigorous adherance to the principles of established technology would require an impeller peculiar to each set of operating conditions in order to maintain optimum efficiency. However, for basic compressor units capable of operation within a range of capabilities, it is common to use impellers which approximate the optimum geometry with a resulting sacrifice in efficiency to reduce cost. Even by doing this, several impeller patterns are required for a family of compressor capable of operation over a wide range.